Molecular Shifting: Metamerism and Tautomerism Quiz

Metamerism occurs in polyvalent functional groups (like ethers, sulfides, or secondary amines) when the total number of carbons is the same, but they are distributed differently on either side of the functional group.

Tautomerism (specifically triad tautomerism) involves the migration of a hydrogen atom from an alpha-carbon to a heteroatom, simultaneously shifting the double bond.

A base removes an alpha-proton to form a resonance-stabilized enolate ion. This intermediate then picks up a proton on the oxygen to complete the transformation to the enol form.

Diethyl ether and methyl propyl ether are metamers. Diethylamine and methylpropylamine are also metamers. Pentan-2-one and Pentan-3-one are metamers of the ketone group. The acid and ester are functional isomers.

The enol form of beta-diketones is stabilized by a six-membered ring formed through an internal hydrogen bond and a conjugated pi system, making the enol much more stable than in simple ketones.

Metamers must have the same functional group. If the functional group changes (e.g., from an ether to an alcohol), the relationship becomes functional isomerism rather than metamerism.

To form an enol, the molecule must be able to lose a proton from the carbon adjacent to the carbonyl (the alpha-carbon). Without an alpha-hydrogen, the 1,3-proton shift cannot occur.

Conjugation and aromaticity (as in phenol) provide resonance stabilization to the enol. Non-polar solvents favor the enol form because it can form internal hydrogen bonds, while polar solvents like water stabilize the polar keto form.

Nitroalkanes with alpha-hydrogens exist in equilibrium with an acidic aci-nitro form (nitronic acid) through a proton shift between carbon and oxygen.

Tautomers are distinct chemical species with different atom connectivity (different positions of nuclei and electrons). Resonance structures differ only in the placement of electrons and are not real, isolable molecules.

The acid protonates the carbonyl oxygen. This increases the electron-withdrawing effect on the alpha-carbon, making the alpha-hydrogen much easier to remove in the next step.

Metamerism requires a polyvalent functional group (bonded to at least two carbons). Ethers, secondary amines, and ketones satisfy this. Alcohols are monovalent and cannot show metamerism.

Glucose exists in equilibrium between its open-chain aldehyde form and its cyclic hemiacetal forms. This interconversion is a type of ring-chain tautomerism.

Tautomerism is a dynamic equilibrium. While catalysts speed up the process, many tautomers interconvert spontaneously at room temperature until they reach a stable ratio.

The enol form (phenol) is stabilized by nearly 145 kJ/mol of aromatic resonance energy. This makes the enol form billions of times more stable than the non-aromatic keto form.